The apparent death in a transporter accident of the Vulcan ambassador T'Pel, in fact the escape of the Romulan agent Sub-Commander Selok after years undercover in the Federation.
In The Next Generation episode Data's Day, told as a letter from Data to Commander Bruce Maddox, the Enterprise-D carries Ambassador T'Pel to a meeting in the Neutral Zone with the Romulan warbird Devoras under Admiral Mendak. She has asked Data probing questions about the ship's defences.
When she beams across, the transporter appears to fail and only traces of organic matter are left on the pad. Data investigates in the manner of Sherlock Holmes, and with Doctor Crusher finds that the residue does not match T'Pel's earlier pattern: it was beamed in to cover a second transporter beam from the Devoras.
Picard pursues and demands her return. Mendak coolly introduces her as Selok, a Romulan officer coming home, and the outgunned Enterprise has to withdraw. The same day sees the wedding of Miles O'Brien and Keiko Ishikawa.